1. Field of the Invention
The present invention relates to a polarization-maintaining optical fiber suitable for an optical transmission system using a laser diode or the like mainly used in the visible wavelength range, and in particular, a polarization-maintaining optical fiber which can be used in the entire wavelength range of 400 to 680 nm and has reduced bending polarization crosstalk and bending loss in the entire wavelength range.
2. Description of the Related Art
Polarization-maintaining optical fiber is a single-mode optical waveguide (single-mode fiber) capable of holding and transmitting a linearly polarized wave.
The polarization-maintaining optical fiber has a schematic configuration including a core, a pair of stress-applying parts provided on both sides of the core, and a cladding surrounding the core and the stress-applying parts.
The stress-applying parts serve to induce birefringence in the core to which light is guided and are a pair of glass regions that are disposed in the cladding so as to be separated from each other with the core interposed therebetween and face each other in the diameter direction of the polarization-maintaining optical fiber.
In addition, the stress-applying part is provided over the entire length in the longitudinal direction of the polarization-maintaining optical fiber.
The thermal expansion coefficient of the glass regions which form the stress-applying parts is different from the thermal expansion coefficient of glass which forms the cladding.
In addition, a polarization-maintaining optical fiber is known in which the dimension (diameter) of a cross section perpendicular to the longitudinal direction in at least one of the glass regions that form stress-applying parts is larger than the diameter of the core.
The polarization-maintaining optical fiber having such a structure is called PANDA (Polarization-maintaining and absorption reducing) type polarization-maintaining optical fiber.
The polarization-maintaining optical fiber is used not only for optical communication but also for connection between optical components having a polarization dependence or optical components for optical transmission, such as an optical fiber grating or an optical fiber coupler.
Taking into consideration the ease of manufacturing, stability of quality, connectivity with a common silica-based optical waveguide fiber, and the like, the polarization-maintaining optical fiber used as an optical component for optical transmission is designed so as to have a single-mode optical waveguide structure in a narrow wavelength range near the wavelength of the light used (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2003-337238, Japanese Unexamined Patent Application, First Publication No. 2008-76655, Fujikura Technology Vol. 85, p. 1-9, issued October, 1993, and Introduction of Fujikura PANDA fiber and Basis of polarization-maintaining fiber (URL:http://www.fujikura.co.jp/products/tele/o_device/data/16pnb04j.pdf)).
Conventional polarization-maintaining optical fibers have been designed so as to have a single-mode optical waveguide structure in a narrow wavelength range near the wavelength used taking into consideration the ease of manufacturing, stability of quality, connectivity with a common silica-based optical waveguide fiber, and the like.
That is, an optical fiber structure which is satisfactory in terms of bending loss and bending polarization crosstalk in the entire wavelength range of 400 to 680 nm is not yet available.
For example, when a conventional polarization-maintaining optical fiber for 400 nm is used at a wavelength (for example, 680 nm) away from the cut-off wavelength to the longer wavelength, if this polarization-maintaining optical fiber is wound 10 times with a small bending diameter (for example, with a diameter of 60 mm), there has been a problem in that the loss or the polarization crosstalk increases.
In addition, in the conventional polarization-maintaining optical fiber, the diameter of the stress-applying part is increased or the distance between the stress-applying parts is decreased so that polarization crosstalk is not increased when the bending diameter is reduced. However, if the diameter is too large, the rate of non-circularity of the cladding is increased.
Then, when the polarization-maintaining optical fiber is connected with a connector or when the polarization-maintaining optical fiber is connected with other optical fibers or optical components, positioning becomes difficult. As a result, there has been a problem in that working efficiency becomes poor.
If the distance between the stress-applying parts is narrowed, a mode field becomes non-circular. As a result, loss of connection with other optical fibers is increased, or the loss of the optical fiber itself is increased due to the influence of the stress-applying parts.
The present invention has been made in view of the above-described situation, and it is an object of the present invention to provide a polarization-maintaining optical fiber capable of suppressing an increase of bending loss and polarization crosstalk.